You are full of it !!! Engage brain before typing. 1 dickhead riding up a trail plugged into his ifag, the other clown belting down also with phones in his ears.... I can tell you what happens next and they both deserve it.
Use a bit of common sense. Listen to what's going on around you and have a bit of respect for other trail users.
To be fair, two way trails are definitely more problematic - and people should take that into account. I started using headphones on the North Shore, as everything runs in on direction, no two way traffic, so this goes a long way to help. That type of terrain tends to make bikes damn noisy so I never had a problem with bikes approaching from behind and not knowing they were around.
Now in Perth the trail centres are really well designed and also run one direction (95%, the odd rider goes the wrong way - no big deal), I keep conscious and mindful of people behind, luckily the longer climbs where someone might be putting the hurt on are more open, passing is easier. Fast downs - again enough bike noise I can hear the odd occasion of a rider behind. I shudder to think how I'll cope with someone on stealth hub and clutch derailleur... so quiet
Racing I think is another story, way too many riders, passing is a total priority and far to many people sorting out positions, and back-forth passing. Not sure if its a rule, but I've never seen the headphones at races in Perth.